Oh goodness! I love this bus set! I look forward to playing with it in the future.
There are so many buses I like though... but these are a few that stood out to me to suggest (I looked for varying types of buses from different eras.)
There's the Mercedes Sprinters (some are renovated vans), like modern City 77 (2010) one (
http://mercedesbenzblogphotodb.files.wo ... over-2.jpg), or the much less appealing classic Sprinter 308D (1995) (
http://upload.wikimedia.org/wikipedia/c ... rinter.jpg) or something in between, the Sprinter Stop/Go / Opstapper (2001) (
http://upload.wikimedia.org/wikipedia/c ... terdam.jpg). And there;s also the Volkswagen Kutsenits City I 1992 (
http://www.busfoto.nl/foto/albums/userp ... 45o_e1.jpg), a very boxy example of a vanbus. Both of these were used for rural areas/intervillage transportation, as a cheaper alternative to actual buses. The buses are fast, have low capacity and have slow loading times, but are indeed quite cheap.
There's also this Japanese blast from the past. The 日本最初のバス 1905
http://upload.wikimedia.org/wikipedia/j ... %82%B9.jpg.
In 1921 this little bus got into service. It's the intervillage OAD (the company) E-409:
http://www.busfoto.nl/foto/albums/userp ... 06/OAD.jpg As you can see, not a very big bus. XD
Not sure if you're looking for trolleybuses, but the Berna 1951 one is an early example with a very simplistic design.
http://en.wikipedia.org/wiki/Berna This Wikipedia also features the 1946 Berna 4 UPO. Another beautiful bus with a charming nose. The 4 UPO is an urban bus, taking it from the fact there's only one name mentioned on the side of it, haha. In general, all Berna buses are nice, in my opinion.
In America, Greyhound is one of the biggest intercity bus service companies. Their buses are often uniquely styled and most certainly differ from European buses. There's the Greyhound GM PD-4103 from 1951:
http://en.wikipedia.org/wiki/GM_PD-4103 (
http://www.motorstown.com/images/gm-pd-4103-08.jpg). The Greyhound 1954 Scenic Cruiser has already been added, and boy what a joy that bus is. I also have a thing for the design of the Greyhound Silversides (
http://www.vintagetrailercamp.com/galle ... .sized.jpg).
The 1959 Mercedes-Benz L312 bus is also a fine bus with some serious looks.
http://www.sva-museumbussen.nl/?page_id=2142 It was used for all kinds of services: intravillage/suburban, as well as even serious touring. Gorgeous bus.
From 1967 - 1988, the standaardstreekbus slowly entered into the Dutch transportation scene. However, the 12m long 1980-1981 MB200 is probably the most characteristic standaardstreekbuses the Netherlands has witnessed (
http://upload.wikimedia.org/wikipedia/c ... do8899.jpg). The term 'standaardstreekbus' literally means 'standard intercity bus,' so you can imagine for what type of service they were used for. In 1988 these buses were followed up by the Den Oudsten B88. These became one of the most common buses seen across The Netherlands. They came in unarticulated, articulated and even trolleybus varieties (In Arnhem). Den Oudsten is a shell though, the specific model buses have different names, and are either DAF or Volvo. Depending on the model and transport company, these buses were used as all different services. For a broad feeling of nostalgia, these buses have to be included in the game. :3
http://nl.wikipedia.org/wiki/Den_Oudsten_B88.
PS: I don't like to stick with the same thing all the time, but also in 1967, but I have to include the Leyland-Den Oudsten "Duikboot" because of its mad nose. (
http://upload.wikimedia.org/wikipedia/c ... GDS_22.JPG). There's also these similar Den Oudsten buses:
https://www.flickr.com/photos/41659357@N07/4240816326/ that went by the same name, but slightly differ from the first batch of "Duikboten". These were used as urban buses.
Also, there is already one Bedford bus in the set included, but there's also this 1968 beauty: the Bedford TDT502, complete with photos and information:
http://www.busfoto.nl/foto/displayimage ... play_media That is one fancy bus, and I presume it's for longer journeys.
There's also the open-top double decker buses, such as the ones that Transports Metropolitans de Barcelona (TMB) use. Never seen open-top double deckers drawn for OpenTTD. Clearly an urban bus used for round-trips (which is a nice difference from services that go from point A to B and back to A.
http://www.flytobarcelona.org/wp-conten ... istic1.jpg
The Irisbus Crealis Neo is the only Irisbus that I'd say you're missing out on right now. This 2008 urban bus features a very recognizable design, and comes in unarticulated, articulated and even trolley.
http://it.wikipedia.org/wiki/Irisbus_Crealis_Neo
I can go on for a while with all kinds of buses. But it really depends on whether I'm sending you enough relevant information and whether my idea of buses to add is similar to what you'd like to add. Regardless, feel free to add whatever buses you like, I absolutely love the idea of the set. ^^